1. Field of the Invention
The present invention is related to processes for reducing nitrogen oxide emissions in combustion systems. More specifically, the present invention provides methods of decreasing the concentration of nitrogen oxides in flue gases emitted to the atmosphere from stationary combustion systems such as boilers, furnaces and incinerators, by selective reduction of nitrogen oxides to molecular nitrogen.
2. Relevant Technology
One of the major problems in modern industrial society is the production of air pollution by a variety of combustion systems, such as boilers, furnaces, engines, incinerators and other combustion sources. One of the oldest recognized air pollution problems is the emission of smoke. In modern boilers and furnaces, emissions of smoke are eliminated or at least greatly reduced by the use of overfire air, or "OFA" technology. In this technology, most of the combustion air goes into the combustion chamber with the fuel, but a portion of the combustion air is added to the flue gases as they come out of the flame, to facilitate combustion of smoke particles and smoke particle precursors.
Other types of air pollutants produced by combustion include particulate emissions, such as fine particles of ash from pulverized coal firing. Still other pollutants are gas-phase (non-particulate) species, such as oxides of sulfur (principally SO.sub.2 and SO.sub.3), carbon monoxide, volatile hydrocarbons, and oxides of nitrogen, mainly NO and NO.sub.2. Both NO and NO.sub.2 are commonly referred to as "NO.sub.x " because they interconvert, the NO initially formed at higher temperature being readily converted to NO.sub.2 at lower temperatures. These nitrogen oxides are the subject of growing concern because of their toxicity and their role as precursors in acid rain and photochemical smog processes. NO.sub.x is emitted by a variety of sources, including mobile sources (such as automobiles, trucks and other mobile systems powered by internal combustion engines), stationary internal combustion engines, and other combustion sources, such as power plant boilers, industrial process furnaces, waste incinerators and the like.
Three principal technologies have been developed and successfully used to control the NO.sub.x emissions from combustion sources such as boilers and furnaces.
The least expensive of the three principal types of NO.sub.x control technologies is combustion modification, i.e., technologies which modify the combustion process so that it produces less NO.sub.x. One example of NO.sub.x control by combustion modification is installation of low NO.sub.x Burners ("LNB") that use fuel and air staging inside the burner. Another example of NO.sub.x control by combustion modification is a technology commonly referred to as "reburning." In the basic reburning process, a fraction of the fuel, typically between 10 and 20% of the total heat input, is injected above the main heat release zone to produce an oxygen deficient reburning zone. Hydrocarbon radicals formed from combustion of the reburning fuel react with NO.sub.x, reducing it partially to molecular nitrogen and partially to NH.sub.3 and HCN. Subsequently, overfire air is added in an amount sufficient to burn out the remaining fuel and to oxidize the NH.sub.3 and HCN in part to molecular nitrogen and in part to NO.sub.x. Because of their low capital and operating costs, combustion modification technologies are usually the technologies first employed when control of NO.sub.x emissions is found to be necessary. However, the extent of NO.sub.x reduction which these technologies provide is limited to about 30-50% for LNB and about 50-60% for reburning.
A more effective alternative to combustion modification is selective catalytic reduction ("SCR"). Currently, SCR is the commercial technology with the highest NO.sub.x control efficiency. In SCR, NO.sub.x is reduced with a nitrogenous reducing agent ("N-agent"), such as ammonia or urea, on the surface of a catalyst. The SCR systems are typically positioned at a temperature of about 700.degree. F. in the exhaust stream. Although SCR can relatively easily achieve 80% NO.sub.x reduction, it is far from an ideal solution for NO.sub.x control. The size of the catalyst bed required to achieve effective NO.sub.x reduction is quite large, and use of this large catalyst, with its related installation and system modification requirements, is expensive to implement. In addition, catalyst deactivation, due to a number of mechanisms, typically limits catalyst life to about four years for coal-fired applications. The spent catalysts are toxic and pose disposal problems.
A third group of NO.sub.x reduction technologies are the Selective Non-Catalytic Reduction ("SNCR") technologies. These technologies, which are somewhat more expensive than combustion modification but which provide additional NO.sub.x control at a lower cost than SCR, involve the injection of a nitrogenous reducing agent ("N-agent"), such as ammonia or urea, into the flue gas at high temperature and under conditions such that a noncatalytic reaction selectively reduces the NO.sub.x to molecular nitrogen. Reduction of the NO.sub.x is considered to be selective because the flue gas being treated contains substantial amounts of molecular oxygen, most of which is not reduced, while the NO.sub.x is reduced.
The first example of NO.sub.x control by an SNCR process was the "Thermal DeNO.sub.x " process in which NH.sub.3 was used for the selective non-catalytic reduction of NO.sub.x at a temperature of 1600.degree. F. to 2000.degree. F. The Thermal DeNO.sub.x process is described in detail in U.S. Pat. No. 3,900,554; to Lyon et al., and in Lyon and Hardy, "Discovery and Development of the Thermal DeNO.sub.x Process," Ind. Eng. Chem. Fundam., 25, 19-24 (1986). Other references, such as U.S. Pat. No. 4,208,386 to Arand et al., and Muzio et. al., Proceedings of the NO.sub.x Control Technology Seminar, EPRI SR-39 Special Report, (February 1976) disclose NO.sub.x reduction by SNCR using urea. In these references, urea or an aqueous urea solution is injected into the hot NO.sub.x -containing flue gases.
The advanced reburning (AR) process, which is an integration of basic reburning and SNCR, is disclosed in U.S. Pat. No. 5,139,755 to Seeker et al. In the AR process, the N-agent is injected into a downstream burnout zone, and the reburning system is adjusted to optimize the NO.sub.x reduction due to the N-agent. By adjusting the reburning fuel injection rate to achieve near stoichiometric conditions in the AR process, instead of the fuel rich conditions normally used for reburning, the CO level is controlled and the temperature window is broadened.
Further enhancements of the AR process are disclosed in U.S. Pat. No. 5,756,059 to Zamansky et al., which describes injection of a reducing agent into the reburning zone and the use of promoters, which enhance NO.sub.x control. The promoters are metal-containing compounds that can be added to the reducing agents, and either one or two stages of reducing agent injection can be employed.
It is well known to those skilled in the art that the range of temperatures in which it is possible to reduce NO.sub.x via conventional SNCR methods is narrow (about 1600-2000.degree. F.). Consequently, the teachings of the prior art that droplets of the selective reducing agent must be directly injected into the hot flue gas means that the injection system must be located in a high temperature section of the boiler, furnace, or other combustion system. In some cases, however, the region of the combustion system in which the injection system must be located is inaccessible, making the use of urea injection for NO.sub.x control impracticable. In other cases, although it is possible to install a system of injectors, installation can be done only when the combustion system is shut down and cooled to ambient temperature. Shutting down a power plant boiler means losing the valuable electricity it would otherwise produce. Similarly, shutting down an industrial furnace can idle the manufacturing facilities which depend on the heat it produces. In general, large combustors are designed for continuous or near continuous operation, making downtime expensive.
Installation of SNCR injectors, even without system shut down, is expensive and responsible for a significant portion of capital costs. Although OFA injectors are available in many combustors, they are not designed for liquid injection and are located at higher temperatures where the SNCR process is ineffective.
Thus, there is a need for a method of using SNCR in boilers and furnaces in which it is mechanically difficult or impossible to install an injector system at the location in the boiler or furnace where the flue gas temperature is appropriate for conventional SNCR NO.sub.x reduction. There is a further need for a method of using SNCR that does not require the expense and downtime of installing an injection system in a high temperature region of the boiler, furnace or other combustion system.